The Best Time to Calculate BMI on Scale

When calculating your body mass index, your first task is to step on a scale. Most basic scales aren’t capable of calculating BMI on their own, as you need to take your height into account, so there is a little math involved. BMI is one way to examine the amount of body fat you have, but it is not always accurate -- if you are a serious athlete, for instance, your extreme muscle mass may skew your results. To calculate an accurate BMI, you need an accurate weight, which may be easier to get at certain times of day.

  1. Time of Day

    • According to Matt Fitzgerald, author of “Racing Weight,” there is no magical time of day to step on the scale. Consistency is more important than specific time -- pick whatever time you want, then stick to it daily or weekly so you have an accurate comparison. Your weight is often the lowest in the morning, as you have been fasting all night, and highest after your largest meal of the day, but Fitzgerald says these fluctuations are normal and that you shouldn’t feel defeated by daily changes.

    Frequency

    • You can weigh yourself as often as you like, but it may not give you an accurate idea of the overall trend in your weight. According to Fitzgerald, you should weigh yourself at least once a month, but weighing in more than once a week can include ups and downs in weight based more on water retention and what you ate for dinner than your weight loss efforts.

    Calculating BMI

    • Once you have an accurate weight, you can calculate your BMI. Take your weight in pounds and divide it by the square of your height in inches. Multiply the result by 703 -- you should get a double-digit number, which is your BMI. If you don’t want to use a single day’s scale reading, weigh yourself at the same time daily for a week, then use the average of those weights in the BMI equation.

    What Does it Mean?

    • The number from your BMI calculation doesn’t mean much on its own. If you are older than 20 years old, the Centers for Disease Control and Prevention classifies your weight status based on a range in BMI. If your BMI is below 18.5, the CDC says you are underweight; 18.5 to 24.9 is the normal weight range and 25.0 to 29.9 is overweight. If your BMI is more than 30, you are obese by CDC standards. If you do fall into the overweight or obese categories, the National Institutes of Health says that losing even 5 to 10 percent of your body weight can drastically improve your health.